A prior Japanese Utility Model Publication (Laid-Open No. 73328/1981) discloses an apparatus for rotating and driving horizontal rack-type conveyor, in which each horizontal rack is provided at it back portion with upper and lower rollers having vertical axes. The rollers are adapted to roll along vertical peripheral surfaces of the respective guide rails, so that the rollers can roll without slipping even at the corners of the guide rail. The vertical load on the rack is born by horizontal rollers provided on the back side of the rack. This horizontal roller has a substantially cylindrical surface so that it inevitably must slip when it rolls along any curved section of the guide rail, because the radially innermost rolling surface and the radially outermost rolling surface of the curved section of the guide rail have different lengths. The horizontal roller can roll smoothly along the straight sections of the guide rail. This slippage generates unfavorable noise and impairs the smooth and light movement of the rack along the curved section of the guide rail, while causing rapid wear of the surfaces of the horizontal rollers and the guide rail surfaces. The friction of the roller surfaces causes noises even when the roller is rolling along the straight section of the guide rail.
In another conventional conveyor including the one which employs the racks, the guide rails for the rollers attached to the upper and lower portions of the rack or like carrier are adapted to roll along upper and lower rails which are formed on the upper and lower sides of the a single guide rail member integrally therewith. Consequently, the distance between the upper and lower rails is undesirably fixed. In addition, there is no freedom of orientation of the structure which supports and mounts guide rail member. When the guide rail member is attached to the rail supporting frame, it is necessary to form bolt holes in the guide rail member to register with bolts projecting from the frame, or else to secure the guide rail member by means of welding. When bolts are used for mounting the guide rail member, the heads of the bolts or the nuts project from the inner surface of the guide rail member and may impede the running of the conveyor. When the mounting is done by welding, the guide rail member may be thermally distorted and impair the smooth running of the conveyor, thereby generating unfavorable noise. Furthermore, replacement of the guide rail member cannot be made easily when it has become worn.